taskq: Don't leak system_delay_taskq on FreeBSD

Adds a missing taskq_destroy() call.

Reported by: Jorgen Lundman <lundman@lundman.net>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #10292

module/os/freebsd/spl/spl_taskq.c

@@ -97,6 +97,7 @@ static void
 system_taskq_fini(void *arg)
 {
 
+	taskq_destroy(system_delay_taskq);
 	taskq_destroy(system_taskq);
 	uma_zdestroy(taskq_zone);
 	tsd_destroy(&taskq_tsd);